1. Field of the Invention
The present invention relates to engine retarders of the compression release type. More particularly, the invention relates to an apparatus and method for modifying the motion of the exhaust valve so as to open the valve more rapidly and at a predetermined time. The invention is particularly adapted for use in engines where the retarder is driven from an exhaust or intake cam.
2. The Prior Art
Engine retarders of the compression release type are well-known in the art. In general, such retarders are designed temporarily to convert an internal combustion engine into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower normally developed by the engine in its powering mode.
The basic design of the compression release engine retarder is disclosed in the Cummins U.S. Pat. No. 3,220,392. That design employs an hydraulic system wherein the motion of a master piston actuated by an intake, exhaust or injector pushrod or rocker arm controls the motion of a slave piston which, in turn, opens the exhaust valve near its top dead center position whereby the work done during the compression stroke of the engine piston is not recovered during the expansion or power stroke but, instead, is dissipated through the engine exhaust and cooling systems.
With compression ignition engines having a fuel injector driven from a third cam on the engine camshaft, it has been found to be desirable to derive the motion for the compression release retarder from the fuel injector pushtube for the cylinder experiencing the compression release event. The fuel injector pushtube is a desirable source of motion both because it peaks very shortly after the top dead center (TDC) position of the piston following the compression stroke and also because the effective stroke of the injector pushtube is completed in a relatively short period, e.g., 25-30 crankangle degrees. Further development of the injector-driven compression release retarder has disclosed the desirability of advancing the timing of the compression release event and this has been accomplished by a timing advance mechanism as disclosed in Custer U.S. Pat. No. 4,398,510. The Custer mechanism automatically decreases the clearance or "lash" in the valve train mechanism so that the motion of the injector pushtube-driven master piston is delivered to the exhaust valve sooner. As the "lash" approaches zero, the motion of the exhaust valve approaches the motion defined by the injector cam. Although the total exhaust valve travel can be increased or decreased by varying the ratio of the diameter of the master and slave pistons (i.e., the "hydraulic ratio"), the elapsed time during which motion occurs is determined by the motion of the master piston which, in turn, is defined by the shape of the fuel injector cam.
Many compression ignition engines employ fuel injection systems which are not driven from the engine camshaft and most spark ignition engines having fuel injection systems do not use an engine camshaft driven fuel injection system. Such engines, commonly known as two-cam engines to distinguish them from the three-cam engines referred to above, utilize a remote intake or exhaust valve pushtube or cam to operate the compression release retarder. The valve motions produced by the intake and exhaust valve cams are similar to each other but significantly different from the motion produced by the injector cam. Typically, exhaust and intake valves require more than 90 crankangle degrees to move from the closed to the fully open position. Additionally, the exhaust cam generates a motion that begins too early, reaches its peak too late and provides a total travel which is too great for optimum retarding performance. Partial compensation for these disadvantages can be effected by increasing the slave piston lash and increasing the hydraulic ratio of the master and slave pistons. Also, as disclosed in Price et al. U.S. Pat. No. 4,485,780, the rate at which the exhaust valve is opened may be increased and the time of opening correspondingly decreased by employing a second master piston driven by an appropriate intake pushtube. Although the time of opening using the invention of the Price et al. U.S. Pat. No. 4,485,780 may be reduced from about 90 to about 50 crankangle degrees, the time is still above that available with an injector cam-driven retarder. As a result, and prior to the present invention, substantially less retarding horsepower can be developed from an exhaust cam-driven retarder than from an injector cam-driven retarder when both are optimized for the same engine.